Hi SlyQQ,
You raise valid points and I'm happy your asking these questions

I can see the difficulties with applying radical acceptance. Radical acceptance is a tool and a choice.
1 How could they cheat on me, the awnser is how could they not it is almost impossible for someone who is BPD to NOT cheat on you furthermore they will blame YOU for it regardless of the circumstances any remorse or regret will be purely a show this is BPD an if you wish to maintain a relationship is something you need to understand it is possible overtime for a BPD individual to LEARN otherwise but it is unnatural for them once you grasp that they are not like you or ' normal " people and that you are in general placing unrealistic expectations upon them life becomes simpler
I can relate SlyQQ. Being cheated on is difficult and painful. I'm sorry. It takes time to work through these difficult experiences and make sense of who's responsible.
I can also relate with being blamed for everything and it's painful and confusing when someone else's actions are attributed to us.
Let's take being blamed for cheating as an example. A defense mechanism that many people with BPD use is "projection"
It's a defense mechanism. Non-disorders people project as well and pwBPD take this to the extreme.
You mention shame and guilt and you would likely feel shame and guilt? Projection is like a game of tag your it! It is taking one's own insecurities and negative actions and feelings and attributing it to someone else. A protective self defense mechanism for the ego. In this case attaching said actions to you ( blame ) and it triggers emotions. It's hard to be blamed for someone's actions when you know your reality is different right?
2 How could they be so cruel /selfish/uncaring Again people with BPD do not trust anyone ( not down deep and certainly not LONG term ) they are waiting/ scared of being crossed by you and in general adopt a strategy of do unto others before they do unto you it is a NATURAL defense mechanism to protect themselves
again regardless of how baseless this may be in fact , fact has little relavence when dealing with numerous
mental conditions
It's hurtful and painful when a loved one shows little to no empathy towards their partners. It's incredibly hard. I'm sorry.
I also think another facet is that a pwBPD have difficulties loving themselves. They feel low self-worth, are insecure and have difficulties
trusting themselves Very painful experiences SlyQQ. Trust is important and I think a way to bridge that gap is by understanding the disorder and the motivations why the mentally ill behave the way that they do.
3 They seemed so genuine they tricked me ... .people with BPD are experts at tricking people not least of all THEMSELVES it is hard not to believe someone ho has convinced themselves on some level what they are saying is basically true
I'm sorry that you were tricked. I can relate. I'm not sure that they are completely convinced with their behaviors. I think some of it goes back to self-awareness? From my personal experience and the person in my life with BPD traits there were rare glimpses at the time that I could see her pain. Being indifferent and emotionally detached now I see and understand. That being said I was hurt, frustrated, fed up with her.
I understand that she has pain ( not her trauma or what caused her trauma, I have speculations and she would have to explore this in therapy ) and the disorder is emotionally arrested development, maladaptive coping mechanisms.
I'm sorry no. I'm not referring to gaslighting.
Can you explain what you mean by dangerous? Was there something that happened to you?
You have many difficult things you are dealing with SlyQQ. Cheated, trust, emotional blackmail, blamed and lied to. I'm sorry this happened to you.
How do you feel?